Vega Instinct end of this year shows TSMC's 7nm is ready to start building real things, so here's how I see 2019 panning out. tl;dr Nvidia will only release xx50/xx60 class GPUs on 7nm in 2019.
AMD Vega 20: end of 2018. This could well put the cat amongst the pigeons for Nvidia in the data center segment.
AMD Navi: end of 2019. Not expected to be anything more than a refined GTX 1080-class product, but if priced right could stir up the low to mid-range consumer market and make Nvidia think a little, but overall nothing to force Nvidia to accelerate their plans.
Nvidia 2050/2050 Ti/2060: middle of 2019. Let TSMC work out the kinks of their 7nm process getting all of AMD's kit going and then benefit from increasing maturity. Consider this an exercise in getting Turing's power draw down and working out their own kinks in shrinking Turing to 7nm.
Could AMD shrink Vega 10 to 7nm also for a refreshed RX Vega line? Perhaps. Is that what Vega 12 is? But I doubt it'll have performance gains sufficient to worry Nvidia into changing tack.
And based on all this conjecture above, Nvidia's 2020 plans will hinge entirely on AMD's Navi. If the 7nm 2050/2050 Ti/2060 experiment goes well, we could see the big RTX Titan on 7nm towards the end of 2019 just to overshadow Navi's launch. We may also get a big data center card to take it to Vega Instinct if does live up to speculation. Otherwise a full 7nm Turing refresh will only come out in 2020 if Navi shows any signs of competition at the upper end of the consumer segment; if Navi offers no competition, 7nm may well wait until Turing's successor.